edit_history: Remove 'prev_rendered_content_version' field.

This commit removes the 'prev_rendered_content_version'
field from:

* the 'edit_history' object within message objects in the
API response of `GET /messages`, `GET /messages/{message_id}`
and `POST /zulip-outgoing-webhook`.
* the 'update_message' event type

as it is an internal server implementation detail not used
by any client.

Note: The field is still stored in the 'edit_history' column
of the 'Message' table as it will be helpful when making
major changes to the markup rendering process.

zerver/lib/event_schema.py

@@ -1674,7 +1674,6 @@ update_message_content_fields: list[tuple[str, object]] = [
     ("is_me_message", bool),
     ("orig_content", str),
     ("orig_rendered_content", str),
-    ("prev_rendered_content_version", int),
 ]
 
 update_message_content_or_embedded_data_fields: list[tuple[str, object]] = [